1. Field of the Invention
The present invention relates to an apparatus for corneal surgery to correct a refractive error by ablating corneal tissue with a laser beam, and more particularly to an apparatus suitable for astigmatic correction.
2. Description of Related Art
An apparatus for corneal surgery to correct an refractive error of an eyeball by ablating a corneal surface with an excimer laser is conventionally known.
However, astigmatic correction, especially myopic astigmatic correction, performed by using this kind of apparatus has a problem that is a hyperopic shift of spherical component often occurs after such a correction. To address this problem, two methods have been suggested: one is to minimize a size of ablation area in a meridian direction for correction so as to reduce affect of the hyperopic shift, and the other is to combine myopic astigmatic correction and hyperopic astigmatic correction in order to correct myopic astigmatism.
The latter method is to estimate a hyperopic shift of the spherical component which will be developed after the correction of the myopic astigmatism so as to combine ablation of each correction in consideration of the estimated hyperopic shift. For example, in the case where a hyperopic shift of 33% is estimated, upon correcting simple myopic astigmatism of which correction amount is S=0 D, C=-3.0 D and A=0.degree., 75% of the correction amount, that is C=-2.25 D and A=0.degree., is removed by ablation for myopic astigmatic correction. As the result, the hyperopic shift of 0.75 D, which accounts for 33%, occurs and the corneal refractive power after the ablation will be S=+0.75 D, C=-0.75 D and A=0.degree.. Here, replacing the sign of the astigmatism with a plus sign, the residual correction amount will be S=0 D, C=+0.75 D and A=90.degree., which is equal to 25% of the correction amount to be corrected by ablation for hyperopic astigmatic correction.
Further, in the case of mixed astigmatism of which spherical equivalent is 0, 37% of the correction amount is achieved by myopic astigmatic correction and 63% is achieved by hyperopic astigmatic correction. For example, in the case where the correction amount is S=+2.0 D, C=-4.0 D and A=0.degree., by performing correction for S=0 D, C=-1.5 D and A=0.degree. which is equal to 37% of the correction amount, C=-4.0 D in this case, a hyperopic shift of 0.5 D which accounts for 33% occurs and the resulting refractive power will be S=+2.5 D, C=-2.5 D and A=0.degree.. Replacing the sign of the astigmatism with a plus sign, the residual correction amount will be S=0 D, C=+2.5 D and A=90.degree., which is equal to 63% of the correction amount to be corrected by hyperopic astigmatic correction.
In the former of the above methods, since the range (the size) where the correction is effected is narrow, there is a problem that the patient may experience halos or glare when, for example, the pupil has dilated at night.
The latter method has been suggested to address the problem arise in the former method. Yet, there is another problem that the hyper shift rate needs to be obtained as an empirical value to estimate hyper shift of the spherical component caused by myopic astigmatic correction. In addition, this method has been applied only for correction of a certain limited range.